A friend of mine, a longtime cyclist, has just been diagnosed with myasthenia gravis. Only symptoms so far are ocular--double vision and a drooping eyelid, corrected with a low dose of mestinon ??).
It's a neurological condition, not curable but usually manageable. About 85 percent of people with his symptoms eventually progress to some degree of generalized involvement, with muscle weakness and fatigue etc. (Google for specific information).
His doctor is encouraging, and there are several chat sites and message boards that seem to indicate that for most people, at least some exercise is possible and beneficial. But it's a rare disease (only about 36,000 cases in the whole U.S.), so information isn't just lying around. Anybody have any experience with MG and cycling or other exercise, or been on long-term mestinon therapy? He's willing to adapt to it (not that he'll have much choice), but he'd like some idea what to expect.

I assume your friend is over 50, maybe quite a bit older.
It is important for him to follow up with a competent neurologist. Mestanon is relatively trouble free and usually works pretty well, although it is symptomatic treatment only! It has some side effects but generally it is well tolerated. It's a neostigmine like drug, which improves muscle strength by bumping up ACH levels but doesn't do anything to the cause of the disease.
The root cause of this weird disease is an autoimmune phenomena, so usually they are put on steroids in the beginning to try supress formation of the abnomal antibodies, then maybe or maybe not weaned off depending on the response. In severe cases dialysis can be used to soak up all or most of the abnormal antibodies produced.
In the younger groups it is often associated with a benign, rarely malignant, tumor in the thymus gland, although in the older age group the thymus gland is more often normal. Usually the face, and upper extremities are most affected, but general overall weakness is usually noted.
To exercise or ride or not!?! Follow his physician's advice, but these peope tend to be pretty good in the morning, then have progressive weakness during the day as a hormone called acetylcholinesterase is depleted at the neuromuscular junction. ACH is the hormone that gives the signal from the nerve to the muscle to make it contract. In myesthenia your body produces antibodies to wipe out the ACH, leaving muscular weakness. The muscle itself is normal, it just can't contract because of lack of ACH.
Think of ACH as having a full gas tank in the morning, and muscular activity depletes it during the day. ACH tends to build up overnight. Vigorous exercise like biking will deplete it sooner, but I have no idea whether his doctor would say to go for it or not. I doubt biking would do any harm other than bring on muscular weakness earlier in the day that would have happened without the muscular activity. (run dry the gas tanks earlier) If worst came to worst and he became too tired out when riding he could always call a taxi to get home. Then back off his level of riding next time. Tailor his own activity to his muscular weakness.
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It all depends on his level of control of the disease. Each case is different and the level of exercise he can tolerate might boil down to a trial and error situation.
Like most autoimmune diseases, nobody know what makes the body screw up to produce the inappropriate antibodies